1. Field of the Invention
This invention relates to a worm reducer for reducing the rotation transmitted to a worm and transmitting the reduced rotation to a worm wheel, and an electric power steering apparatus provided with the worm reducer.
2. Description of Related Art
In recent years, a vehicle is provided with an electric power steering apparatus for assisting steering by reducing the rotation of an electric motor prepared as a driving source for steering assistance through a reducing mechanism and transmitting the reduced rotation to a steering mechanism. Such an electric power steering apparatus adopts a worm reducer consisting of a worm and a worm wheel as a reducing mechanism for transmitting steering assistant force to a steering shaft by the electric motor.
FIG. 9 is a view showing a conventional worm reducer. FIG. 9(a) is a view showing a worm and FIG. 9(b) is a view showing the peripheral surface of a worm wheel. In FIG. 9(a), on the outer periphery of a worm 110 made of metal, screw faces 112 are formed to protrude from the outer peripheral surface 111 of the worm 110. On the other hand, in FIG. 9(b), on the peripheral surface of the worm wheel 120 made of synthetic resin, worm wheel teeth 121 in mesh with the screw faces 112 of the worm 110 are formed. The worm reducer used as a reducing mechanism of the electric power steering apparatus, which must has a reversely operable structure not permitting self-locking, provides large lead angles of the worm 110 and the worm wheel 120.
Further, in the conventional worm reducer, as seen from FIG. 9(b), the worm wheel teeth 121 on the peripheral surface of the worm wheel 120 are generally formed using a hob-cutter 130 as shown in FIG. 10. The hob-cutter 130 is provided with cutting blades 132 each having a slit 131 for tooth-cutting on the outer periphery. The peripheral surface of the worm wheel 120 is tooth-cut by the cutting blades 132 to form the worm wheel teeth 121.
If the diameter D1 of the hob-cutter 130 for making the worm wheel teeth 121 is equal to the diameter D2 of the worm 110 in mesh with the worm wheel 120, the meshing contact area between the worm 110 and the worm wheel 120 is large. As the diameter D1 of the hob-cutter 130 becomes larger than the diameter D2 of the worm 110, it becomes small. For this reason, in the case of the worm reducer requiring high endurance, generally, the meshing contact area between the worm 110 and the worm wheel 120 is increased to reduce the face pressure loaded on the tooth face.
However, in recent years, in the conventional worm reducer shown in FIG. 9, the strength of the synthetic resin constituting the worm wheel 120 was increased to improve the endurance. Correspondingly, shortage in the strength of the worm 110 occurred. The strength of the worm 110 can be increased by increasing the diameter D2 of the worm 110. In the conventional worm reducer, however, in view of structure, it was difficult to increase the diameter D2 of the worm 110 while keeping the predetermined wheel diameter of the worm wheel 120 and lead angle of the reversely operable worm 110.
Further, it is desirable that the hob-cutter 130 employed for making the worm wheel teeth 121 of the worm wheel 120 has the same diameter as that of the worm 110 in order to increase the meshing contact area between the worm 110 and the worm wheel 120. In this case, the cutting blades 132 of the hob-cutter 130, which have the slits 131, are weak in strength and so may be broken. In the conventional worm reducer, therefore, in making the tooth faces of the worm wheel 120, in view of strength, the hob-cutter 130 having a larger diameter than that of the worm 110 was required. Thus, as described above, since the diameter D1 of the hob-cutter 130 becomes larger than that of the worm 110, the meshing contact area between the worm 110 and the worm wheel 120 is decreased. This advantageously leads to an increase in the face pressure loaded on the tooth face.
Further, the conventional worm reducer is an orthogonal type in which the axial line of the rotary shaft of the electric motor coaxially connected to the worm 110 and the axial line of the steering shaft into which the worm wheel 120 is press-fitted are at right angles to each other in their non-crossing state. Therefore, the mounting layout of the electric motor was limited. Further, with progress of the high output power of the electric power steering apparatus in recent years, the worm wheel teeth 121 in mesh with the screw faces 112 of the worm 110 are required to have high strength. However, in the conventional orthogonal type worm reducer, it was difficult to assure sufficient strength of the worm wheel teeth 121.
Further, in the conventional worm reducer, where the tooth faces of the worm wheel 120 are formed using the hob-cutter 130 having the larger diameter than that of the worm 110, the lead angle of the hob-cutter 130 is different from that of the worm 110. Therefore, the tooth-cutting was made with their corrected angles so that the lead angle of the worm wheel 120 is fit to that of the worm 110 at pitch points. However, in these worm 110 and worm wheel 120, there was fear of generating interference at the area other than the pitch points. In order to avoid interference occurring in the area other than the pitch points, proposed was an obliquely crossing type worm reducer employing the worm wheel in which the tooth marks of the worm wheel teeth 121 remain unchanged in the tooth lines. In this case, however, the contacts at the mutual meshing area are point contacts and so the face pressure in the meshing area becomes high. As a result, the endurance of the worm reducer will be lowered.